Environment

VB.net
.NET core 3.1

Aim

Define a Text.Encoding instance which defines Shift-JIS and UTF-8 without BOM.

References

Code

Dim UTF8 as Boolean

'(...)

Dim enc As System.Text.Encoding

If UTF8 Then
    'UTF-8 encoding without BOM
    enc = New System.Text.UTF8Encoding(False)
Else
    'Shift-JIS encoding
    System.Text.Encoding.RegisterProvider(System.Text.CodePagesEncodingProvider.Instance)
    Dim encFallBack As System.Text.DecoderReplacementFallback = New System.Text.DecoderReplacementFallback("?")
    enc = System.Text.Encoding.GetEncoding("shift_jis", System.Text.EncoderFallback.ReplacementFallback, encFallBack)
End If

Note

New System.Text.UTF8Encoding(False) defines UTF-8 without BOM.
- System.Text.Encoding.UTF8 seems good as well, but it returns UTF-8 with BOM, which is equivalent to New System.Text.UTF8Encoding(true).
  
  cf. Encoding.UTF8 Property (System.Text) | Microsoft Docs
On .Net Frameworks, the simple code enc = System.Text.Encoding.GetEncoding("shift_jis") may work. However, it is not the case on .Net Core nor .Net, and RegisterProvider is needed.
Plus, the code above replaces unencodable characters to "?", owing toencFallBack.